How to write method that has argument implementing an interface in Java

Question

I have an interface called ListInterface<T> that has the method join:

public interface ListInterface<T> {
     /**
     * Join a new list to the original list.
     * @param otherList The new list to be joined.
     * @return Original list with appended part from the new list.   
     * @throws IllegalArgumentException.
     */
     public ListInterface<T> join(ListInterface<T> otherList) throws IllegalArgumentException;
}

Two classes DoubleLinkedList and MyArrayList implement this interface. So in DoubleLinkedList, I need to write join like this:

public ListInterface<T> join(DoubleLinkedList<T> otherList) throws IllegalArgumentException {
    (...)
}

And in MyArrayList:

public ListInterface<T> join(MyArrayList<T> otherList) throws IllegalArgumentException {
    (...)
}

But this is not possible and the type of arguments must be ListInterface<T>. If I change the method signatures in these classes to public ListInterface<T> join(ListInterface<T> otherList), then I can't use other specific methods on DoubleLinkedList or MyArrayList anymore. How should I change the method signature in ListInterface in order to fix this?

John Kugelman · Accepted Answer · 2018-04-29 16:39:08Z

4

How should I change the method signature in ListInterface in order to fix this?

You don't. ListInterface's method signature is correct as is.

If I change the method signatures in these classes to public ListInterface<T> join(ListInterface<T> otherList), then I can't use other specific methods on DoubleLinkedList or MyArrayList anymore.

That is correct. You should not be using any subclass methods because otherList may not be the same type of list as this. The user might try to join a DoubleLinkedList to a MyArrayList, yes?

Use the interface methods only.

answered Apr 29, 2018 at 16:39

John Kugelman

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